An active antenna system (AAS) antenna array has a capability of performing dynamic adaptive beamforming (BF) in a vertical direction and a horizontal direction. This is because that multiple array elements in the vertical direction and the horizontal direction in the AAS antenna array (which generally is a planar array or a three-dimensional columnar array) can perform dynamic adaptive weighting, so as to implement so-called “three-dimensional BF”. In a frequency division duplexing (FDD) system, in order to obtain better BF performance, a user equipment (UE) is required to perform a measurement on channel state information (CSI) of the AAS antenna array and then feed back a measurement result to a base station (BS).
To effectively obtain a CSI measurement result, when a measurement is performed by using a CSI-RS, a different vertical direction weight may be used for each vertical array in the AAS antenna array, so as to obtain virtual horizontal arrays pointing to different vertical directions (each virtual horizontal array may also be considered as a virtual transmission node). Each virtual horizontal array may use one set of CSI-RS resources (or different forms of virtual arrays may be formed in another manner, and each virtual array uses one set of CSI-RS resources). A BS generally configures multiple sets of CSI-RS resources for a UE, and the UE may perform a measurement on these CSI-RS resources.
It is assumed that each virtual array (for example, a virtual horizontal array) uses one set of CSI-RS resources, and a BS (for example, an evolved NodeB (eNB)) notifies a UE of M sets of CSI-RS resources. The UE selects the most preferred N sets of CSI-RS resources from the M sets of CSI-RS resources (for example, according to an RSRP criterion), and then reports an identity (ID) of a corresponding CSI-RS resource to the BS. As a result, during a next measurement, the BS configures corresponding CSI-RS resources reported by the UE for the UE, and the UE performs a measurement on these CSI-RS resources.
A disadvantage of the foregoing method is that, if the number M of sets of CSI-RS resources configured by a BS for a UE is relatively large, and the number N of sets of preferred CSI-RS resources that the UE needs to feed back is also relatively large, feedback overhead is relatively large. For example, assuming that M=10 and N=2, a combinatorial number CMN of feedback CSI-RS resources is C102=45, and then the number of bits used when the UE reports a measurement result to the BS reaches eight bits. If M and N further increase, the number of bits used during feedback is larger.